Power amplifiers have a degree of non linearity in their transfer characteristic. This non linearity results in the distortion of the output signal so that it is no longer a perfect replica of an input signal. This distortion produces various signal components known as intermodulation products. Intermodulation products are undesirable because they cause interference, cross talk and other deleterious effects on the performance of the system employing the amplifier. Feedforward amplification for reduction of distortion is known and has been successfully applied to radio frequency amplifiers. Feedforward amplifiers typically separate out the distortion generated by a power amplifier and adds the distortion back into the power amplifier's output with gain, phase and delay adjusted for maximum cancellation.
A known feedforward approach includes the use of a test signal or pilot signal that is injected into the main signal path of the power amplifier. The magnitude of the pilot signal when detected at the amplifier output, is used by an automatic control circuit to adjust the gain and phase of signals in an error correction path of the amplifier in order to eliminate both the pilot and the distortion introduced by the power amplifier. The problem with some such amplifiers is that they inject only a single pilot tone which fails to provide a wide bandwidth solution to intermodulation product cancellation. As a result, other useful types of feedforward amplifier networks include the use of a frequency swept pilot tone signal that is continuously variable.
One such known system generally shown in FIG. 1 is disclosed in U.S. Pat. No. 5,130,663 assigned to instant assignee. In such a system, an input signal which may consist of a plurality of RF carriers, is routed between two signal paths by a directional coupler 2. In the main signal path, the input signal is amplified in main amplifier 4 and directed to output 6 through directional coupler 8, directional coupler 10, delay 12, and directional couplers 14 and 16. The input signal is delayed by delay circuit 18 in the feedforward signal path and phase and gain adjusted by the phase and gain adjuster 20 without distortion being introduced. The delay block 18 is set to compensate for the signal delay through the main amplifier and directional coupler 10. Directional couplers 10 and 22 permit a portion of the signal having a distortion component to be combined with the feedforward signal. If the amplitude and the phase of the feedforward input signal is properly adjusted the carrier components of the amplified signal from the directional coupler 10 will cancel the carrier components of the feedforward input signal, resulting in an error signal at the output of directional coupler 22. This process is often referred to as carrier cancellation.
The amplitude and the phase of the error signal is modified in amplitude and phase adjuster 24, amplified in error amplifier 26 and routed to directional coupler 14 where it is subtracted from the output of the main amplifier 4 via directional coupler 10 and delay 12. The time delay of delay 12 is set to compensate for the signal delay through directional coupler 22, gain and phase adjuster 24 and error amplifier 26. If the amplitude and the phase of the error signal is properly adjusted, the distortion component of the main signal path will be canceled, resulting in a clean signal at the main signal path output 6. To ensure proper distortion cancellation, pilot tone generator 28 produces a frequency swept pilot tone signal 30 which is injected into the path of the input signal via directional coupler 8 and delivered to the main amplifier 4. The amplitude of the pilot tone signal is controlled to be equal to the level of distortion components generated by the main amplifier 4. Consequently the error signal at the output of the directional coupler 22 is substantially representative of the distortion component introduced by the main amplifier 4 and the pilot tone signal 30.
To determine the extent of pilot tone cancellation, a pilot tone detector 32, which may be a narrow band pilot tone receiver, is phase locked or synchronized with the pilot tone generator 28 via a local oscillator signal 34. The pilot tone generator 28 and pilot tone detector 32 operate off the same reference signal 34 and can identify pilot tone signals despite the presence of additional signals on the main amplifier output path provided the input signal frequencies are spread far enough apart to allow detection of the pilot tone signal between the carriers.
However, a problem arises with such systems when the input signal includes carriers that are close together in frequency and amplitude so that the power from the carriers are uniformly spread. As a result, carrier power can overwhelm the pilot tone receiver. Such problems may arise when the system is used in radiotelephone systems employing code division multiple access (CDMA) channels. With such channels, the pilot tone detector may fail to suitably synchronize for the more stringent and closer carrier spectrum since the system does not provide for the gaps that would otherwise occur among the varying carriers of the input signal. Hence, a system such as the above may have difficulty during the presence of the full bandwidth of the multi-channel CDMA input signal.
Also, such systems typically utilize complex frequency synthesizers for the pilot generator and pilot tone detectors and typically employ phase locked loops for locking the common reference signal between the pilot tone generator and pilot tone detector. Such designs may introduce additional error over time when the reference signal frequency is swept since differing divide ratios in the pilot tone generator and pilot tone detector may cause the phase lock loop frequencies to move out of alignment. Moreover, a demand exists for smaller amplifiers that are more cost effective to allow additional features and circuitry to be placed on the same printed circuit boards.
Another type of feedforward amplifier is disclosed in U.S. Pat. No. 5,528,196. Such a system employs a pilot signal which lies outside of the operating frequency band but within the pass band of the system and generally uses a fixed frequency pilot tone signal. This design utilizes a mixer arrangement instead of a phase lock loop arrangement but also utilizes a bandpass filter for the pilot tone detector which can prevent sweeping of the pilot signal frequency across the necessary band. Also, it is desirable to use pilot signals that are spread across the operating frequency band to get a more accurate representation of how the linear amplifier performs (distorts) when amplifying desired signals.
Moreover, current pilot tone alignment schemes generally require a substantial amount of radio frequency hardware and elaborate control software which leads to the need for high end microprocessors to implement the control software. The sheer size of the circuitry creates problems for future generation products that require 200% to 400% size reduction.
Therefore, there exists a need for a feedforward amplifier network for reducing distortion generated by a power amplifier that utilizes pilot tone cancellation techniques which can provide improved distortion reduction for systems having input signals where carriers are relatively close together in frequency. It would also be advantageous if such a system and method detected and reduced distortion with pilot tones that are within a same frequency band as an input signal to the power amplifier. In addition, such a system should afford cost advantages and allow smaller size amplifiers to be implemented for desired systems.